How can I alter my photos to make them be small enough for my cell phone to support?

I put some of my photos that were on my computer on my memory stick for my cell phone. But when I tried to open them it said that the photos were not supported. I think it is because they were too large but im not sure. Is there a way that I can change my photos so they will fit on my phone?How can I alter my photos to make them be small enough for my cell phone to support?
If you have a camera phone, send your email a picture message as you would send one to a person with a picture of anything. Take that image from your email, open it on an image editing program such as Photoshop and find out the size proportions. Open your desired image on Photoshop, re-size it to your liking and email it back to your phone number, the return address you got when you sent the picture message.