perplexed

so i've been trying to figure out why my photo-banner looks all pixelated and jumbled, when all my other photos are fine. i thought it was because it was resized. so i resized it properly in photoshop, and it looked great- but the second i upload it, it goes all goofy again. *frown*. i want my pics clear damnit. all the other ones work fine. anyone have any ideas?