I'm interested in hacking one of those digital picture frames (like you see for sale at Walmart) so it fetches and displays an image off the web every 5 minutes or so. (I'm going to have it load a current image.) Any ideas on how to get started?
I don't think any of them have a form of internet connectivity. I think that would be your first goal. I would start by looking at microcontrollers. The Arduino being a popular one, or the Atmel AVR chips it is based on. The Arduino has at least one add-on called an ethernet shield which you could use to gain network connectivity. You'd have to author some code that is capable of connecting to the site and downloading what you want, depending on the chip storage capacity and your coding ability, it might be very simplistic or quite sophisticated.
Next, you have to have some way of getting the device to use the downloaded images. I don't know if the picture frames use a USB connection to load images onto internal memory, or rely on some sort of flash memory card. If it simply reads an SD card, I don't know how you would be able to hack that, unless you inserted your device between the card reader in the picture frame and your own on-board memory. If it's a USB device, you could make your device emulate a USB flash memory drive.
I'm sure there are many ways to hack these things, you might find some more suggestions on instructables.com, which has numerous microcontroller projects.
Just a note, the Arduino and Atmel AVR chips are a lot of fun to work with, but the learning curve can be a challenge. You can write code for them in C or assembler, or one of the many proprietary languages for microcontrollers. Also Atmel isn't the only choice, there are also PicAxe and BasicX and others.
Robotshop.com has a good selection of controllers, but check around on the web, as there are literally hundreds of choices and vendors.