Hi rosamundjohn,
The maximum size of picture you are likely to need for showing on screen is 1024 x 768 pixels. That is only three quarters of a megapixel. Even that is larger than many screens and you may find 640 x 480 is adequate. Digital cameras these days are all capable of much larger images than that but most will have a setting to save smaller images (eg 640 x 480 pixels).
Other features to consider:
If your subject matter is likely to be small and close-up, then you might find a Macro setting on the camera is useful.
You might find an optical zoom lens useful as well to enable you to fill the frame with your subject matter.
These two features are found on all but the cheapest of cameras.
You will also have to master some software tasks, particularly cropping and basic re-sizing (re-sampling) of saved images to produce smaller picures for particular purposes. You can do that with MS Paint that comes with Windows, download one of the many free programs available, or use whatever is supplied on the utilities disc when you buy your camera.