This KB Article refers specifically to iTunes
Q: I need to change the image of my podcast, how do it do it?
A: Everytime you change your podcast image/logo, the new image must have a filename that has not been used before. For example, if your current artwork is podcast-logo.jpg, the new image must be named podcast-logo1.jpg and the filename of the image must match accordingly. The filename must change because a lot of directory websites such as iTunes will cache your image, so the only way to force an image change is to change the file name and update the podcast directory. All podcast images must now be 1400x1400 to allow for iPad users.