2D Level loading from bitmaps

For starters, you have to know how to open and read file data. I'll leave that part up to you. You will also need information on the file-format for the type of image you are using. There are plenty of sources on the web. Here's one http://www.wotsit.org/

Basically, you open the file, start reading data and store the important stuff, maybe in an array or something. A lot of the data is just for identifiy the type of file, getting a description of the image (like sizes and bit-depth) and also any compression information.

[edit]Damn! Posted link to wrong site. Damn thing nearly crashed my whole computer![/edit]
[edit] Ok, fixed the link [/edit]

Rather than loading an image and then interpreting pixel colours, why not just create a plain data file that defines what you want where?

Creating an editor might take a little while, but it's a lot better than using a pain program for level design.

I agree. Create your levels in flat text files (at least to start with). Create a level editor while or before you create your game, it makes developing your game much, much simpler. With the last game I developed, I hand-edited the text file for each level, until it just got too cumbersome to remember what each entity code stood for. So then I just bit the bullet and created a level editor and that was a huge time and frustration saver.

Cheers