If you, like me, only want images with transparent background then you should be looking for .png or .gif images with transparent background - although this is obvious, you might not be aware of the best way to look for them so here's a tip for you: look for game CG. A good site for that would be hitomi. Alternatively, if you find a image that you really enjoy but has a background , you can try your luck with photoshop to crop them - the easiest ones to do this are the ones with simple backgrounds such as this one. I use a couple of photoshop actions I made to automate some actions such as cropping empty spaces, making portraits from body images with 2 clicks and saving images in batch as .png, among other things.
Another thing to consider is to use the Portrait Pack Editor by Kyler2 to easily tag the images so that you may share your packs with the community and it will be compatible with the Improved Random Portraits mod so everyone will be able to enjoy them.