I've recently uploaded this seasonal gouache painting of a Westie (Westhighland Terrier) with a robin to my Imagekind gallery, NajART, where you can purchase it as a print. I've also had a go at making a T-shirt design which can be purchased from Depictions gallery at RedBubble. The border shows some of the quick effects you can apply by using Frame Mask from the drop down edit menu on Faststone Image viewer.
If you want to add your art to one of the many online galleries on the net, you will find that they often have different and varying requirements for the resolution and size of the images you submit. In fact the variations in requirements between the different sites can be quite frustrating. The easiest and quickest way I have found to prepare images for such sites is to use the free open source Faststone Image Viewer software I have mentioned before in movie maker posts. You can very efficiently set the image size, print output size and dpi. Simply select your image and go to Edit>Resize/Resample where you will be able set the various options. You can also save your work in various file formats e.g. TIFF, PNG, JPEG, GIF etc. If you are interested in online galleries, there's some interesting articles on emptyeasel.com: comparison chart and comparing features, traffic, pricing and more, for example.
I wouldn't have high expectations of selling art directly from just listing(s) on these online gallery sites. In many cases there are 1000's of images of artwork for buyers to browse. Some sites seem to make inflated claims about their search engine visibility - they may not be any more visible than your own web site, if you've got that well-optimized. If you are serious about selling, apart from good artwork, you need to have your listing(s) optimized by good tagging, titles and descriptions. You also probably need to use your website or blog and social networking platforms to promote your listing(s). This is one reason why I choose not to pay for online gallery services. (Ebay fees I can stomach - with the traffic eBay get, you can more or less be assured potential buyers will browse your work!) There is no harm, though, in "putting yourself about a bit" and joining some online galleries. It's one way of promoting your art to a wider audience, even better if you can place links to your website/blog, and you may even get the odd sale or commission.