Firstly you would need to see what the resolution of the digital photo frame is in pixels - here it's better to get the highest resolution you can afford, as the image will be clearer and less "pixellated"; also go for the largest screen size you can afford too, so that people will be able to see what's displayed on the frame easier, especially from a distance.
The resolution in pixels is normally given as the number of pixels (dots) across times the number of pixels downwards, e.g. cheaper frames might be 480 x 234 - so 480 dots across and 234 dots downwards, whereas more expensive ones could have a higher resolution, e.g. 800 x 600. Do bear in mind though that most digital photo frames have poorer resolution than the majority of computer monitors.
Another thing to bear in mind is aspect ratio - e.g. if you want standard or "widescreen" - 480 x 234 and 800 x 480 frames will be more "widescreen" whereas 800 x 600 will give you a standard "4:3" aspect ratio.
Once you've got those details sorted and bought your frame, you'll then want to prepare the images to display on it so that they are exactly the same size in pixels as your frame - this will mean that each image will exactly fill the entire contents of the frame, and the frame won't have to try and "resize" the images to fill the space (which could risk them looking blurry, "blocky" or pixellated, depending on the maths that the software in the digital photo frame uses for resizing the images - this could be especially noticeable on things like text).
You'll probably want to save images in JPEG format for use on a digital photo frame too (.jpg file extension) - there's a possibility that the digital photo frame may accept other image formats too - but you can almost certainly guarantee that every digital photo frame out there will display JPEG images, as it's the standard format that digital cameras take pictures in.
If there's a specific order that you want the images to appear in, then you may want to be careful in what filenames you give each image too, particularly if the software in the frame doesn't give you any options for ordering the images once you've put them on there.
e.g. if you have got, say, 20 images that you want on there, and you want them in a specific order, then you could start the filename of each image with a number from "01" to "20" - so your first image might be called "01-SpecialOffers.jpg" and your last image might be called "20-SalonPhoto.jpg" for example. And (assuming that the photo frame displays images in ascending order), then you'd be seeing the image starting "01" first, and "20" last, with all the other images shown in between in ascending numerical order.
Then there's the question of getting the images actually on to the digital photo frame. And here, I think that most digital photo frames will take SD (Secure Digital) memory cards - or will have a USB connection. So it will be a question of either getting an SD card and an SD card reader (your computer or laptop may have a card reader that takes SD cards built in - so it's worth checking before going out and buying another one) - or connecting the digital photo frame to your computer using a USB cable...