Pretty Isn't Enough to Sell Well
OK, so you make absolutely stunning images. Why aren't they selling? Why can't you make the perfectly attainable goal of $500/month?
1. Image Edges
Re-work your images to that they are appropriate for, and fit, the products they are on.
Walk around stores and shops selling t-shirts. How many have images that are sharp edged squares? No matter how wonderful the image it needs to fit well with the product. For t-shirts that means doing something with the edges - soften them, rounder (oval) shape, add a well defined frame, add text that is differently aligned, remove the background, be creative and use anything that gets away from the "piece of paper slapped on a T-shirt look.
There are many possible solutions for the "rectangular image" problem. Here is one
And another good resource
http://myjanee.com/tutorials.htm#photoart You can get some ideas from
http://www.autofx.com/detail_pages/pgedetail.html Some would work wonderfully on even the dark shirts, some would need some adjustment to work, some don't really solve the base problem - but neat stuff. Don't reject these tutorials just because you aren't using photoshop. Most graphics applications work in similar ways so use the intstructions to give you ideas.
Of course this isn't an absolute "rule" - in design there is no such thing. Nevertheless most images benefit greatly when adjustments are made for apparel.
2. Image Shape
Just as images need to be adjusted for apparel, most designs need to be adjusted for the various products to look their best. Don't even offer products when it is obvious that the image does not fit.
It is rare that any image, including "just text," works well on all products. Look at these examples and see how changes in the base image improve the look on some items - but then look bad on others. Also note the effect of the items with poor fit. They are distracting and generally give an unprofessional feel to the offerings.
Even when an image can be made to fit a product through resizing very often both efficiency and the quality will be improved if you create a separate image to fit that product. This mostly involves the bleed products because the closeness of the product edges to the image make shape more obvious. A common practice is to put square shirt images on round buttons. This can require quite a bit of resizing and often looks like, well a square image stuck on a round button. Consider a separate image with appropriate margins for the button, and take advantage of the full bleed to give it a fully colored background. This not only gives a better look but means you can apply the image to all the round products without resizing.
3. Descriptive Text
Write meaningful names and descriptions. If you can't, hire someone - trade/barter whatever it takes. If you decide not to, then don't complain about lack of sales. No one can buy your wonderful images if they don't know they exist. What kind of words to use? First, the search engines can't see your images, and they (mostly) don't show them to the customer. So you need to describe your images in words. And the way you should describe them is so that the customer can find them. Which do you think a customer would use to find your products "original unique art on t-shirts" or "colorful dragons flying through fantasy landscapes on t-shirts" ? The more specific you are in your descriptions the more likely that your items will be what the customer is searching for. There are two things I use in trying to write (1) I'm on the telephone telling my friend about this great new design. What words to I use to convey what is special about the design? (2) I'm trying to find the shops who are my competitors, other people who are creating and selling designs that are similar to my own. What words to I put into the search field to try to find them?
4. Image Tags
Most POD systems have a "keyword" field. Sometimes it is related to a particular product, sometimes to each image. In either case it is used for the internal search engine and you likely will not appear in that POD's listings if you don't use them. Tag your images accurately. If you tag inaccurately you hurt your ranking. It isn't a matter of "well at least people will see it" Bad tagging can make make the remaining good tags less relevant and sink your image in the rankings.
5. More Tips
And do the work. You get out of it what you put into it. It takes a long time to get a new business off the ground (a year or more), and you have started with the HUGE advantage of only needing to spend your TIME to do it. Make use of that opportunity.
For a detailed description of various print processes used by custom garment producers see the article "What is Digital Garment Printing?"