You can upload any photo dimensions you want as long as the file size does not exceed 4 MB. However, your photos will load much faster when a visitor views them if you reduce the size before uploading.
I would recommend resizing your photos to 800 x 600 pixels before uploading and reducing the quality to 80 if you have that option. This usually creates a file size between 150K to 200K. (Never save over your original photos when resizing. Create a separate folder called “blog” within the folder where your photos are stored to keep the resized photos separate.)
Most visitors will be able to see a 800 x 600 pixel photo without having to scroll up and down.
If you don’t have a photo editor, Picasa is a free photo editor. Use the helps within the program to learn how to resize photos or search for something like “how to resize photos picasa” from Google.