On all MyWikis wikis, the logo should be uploaded to the File:Wiki.png page on your wiki.
If you are using the Vector skin, its dimensions should be no greater than 155px by 155px. If you are using the MonoBook skin, its dimensions should be no greater than 135px by 155px. If you are using the Chameleon skin (addable by request), its dimensions should be horizontal but can be as large as 1200px by 200px.