The file size needs to be under 500 kb, and the photo width needs to be under 600 pixels, if I remember correctly. If the photo is too large, it won't upload. (There are some free online programs for re-sizing photos if you don't have something on your computer for that.)
Looking forward to seeing your pic
Working with digital images is like one big, giant game of Fizzbin. There are many interdependent, esoteric rules which make it difficult seem difficult to understand. It's not really hard, per se. It's just requires detail oriented thinking.
This is why concepts like "DPI" gain such a mythical status. The truth is, DPI means very little to the average person. It only really applies to professional printing such as magazines or newspapers. To the average person sitting in front of his home computer, it means almost nothing.
The most important concept is the number of pixels in the image. Is it a "small" image of 100 x 100 pixels? Is it a "large" image of 1,000 x 1,000 pixels?
A large 1,000 pixel image might look better on a computer screen or printed on paper but, if that image will be scaled or cropped to a different size, you might be wasting space. A 1,000 pixel image takes up one hundred times more space than a 100 pixel image. That means that a 1,000 pixel image takes 100 times longer to download and it takes 100 times more disk space to store. When you're talking about a website owner who might have to pay his service provider based on bandwidth or storage space (or both) it could mean the difference between staying on the air or going out of business.
(We could also talk about compression and bit depth but that's a whole other game of Fizzbin. )
That's why Teddy Talk has limits on the size of pictures that people upload. Users don't want to wait a long time for images to upload/download. The owner doesn't want to pay too much money. The limit is there to keep things in balance.
So, when the website sends you back a message saying, "Picture too large," that's what it means. You most likely don't need to alter the DPI setting. You'll probably have better luck changing the size of the image in pixels. (i.e. "scaling" the image.) Like Becky, "Dangerbears" says, there is a 600 pixel size limit.
If you are using Mac OS X, the picture viewer program called "Preview" can scale images quickly and easily. It's under the menu, "Tools > Adjust Size..." Just change the size and "File > Save As..." (Don't save over your original image! )
Ubuntu's "PicView" can do basic scaling and rotation, as well. I'm sure Microshaft has one, too, but I've never bothered to look.