I generally use images.google.com and then pick the biggest and clearest up to about 500 x 500 pixels.
If I can't find the CD or LP image that way, I try eBay, take a screenshot, and then use Paint or something similar to crop out the image.
If that doesn't work, I get out the camera and take a picture of the CD cover art or LP cover.
If it is something I recorded live, myself, I try to use a meaningful picture. For example, when I record my son's high school choir I usually try to take a picture of that night's performance and lay some text over it like "John Smith High School Winter Concert 2011".
I've also created year-by-year collections of songs that were in the top 100 that year, including any other single that I really like from that year. For those, I found an image of a real jukebox from that year and overlayed "1961" (for example) over that image as an album cover.
Brian