It's a lot like Photoshop but completely free and more user-friendly. You can upload pictures from your computer or through Flickr, Picasa, the web, etc. and then edit them.
One of the funnest tools they have is EV Collage and it makes it really easy to do a banner for your blog.
So, to make a web banner (ahem, Sis), go to the link above and click on "Upload Photos" then follow the guide to upload from Facebook, your computer, etc.
Under the Basic tab there is an EZ Collage button. Click on this and then click "don't save."
Choose the layout like 1 x 3 square (or whatever one you want). Basically, you are making all your pictures into one big picture that will be saved and then posted on your blog.
You can change the background color, spacing between photos and proportions.
Load each photo
Then you can click on the customize tab at the top to further edit. If you're happy with it as is, click save.
To type text over your photos, click the decorate tab. You can play with the size, font, background (I like transparent), etc. You can change the dimensions of the text box by clicking and dragging the little blue squares around. You can move the it around your photo by clicking in the center and dragging it to the desired location.
Fun tip: If want to smooth away blemishes and wrinkles, try the beautify tab. Also, the sharpen tab under the beautify tab makes your edges look more defined. And the animations tab is pretty fun as well (add the little butterfly like the Pioneer Woman)!
Once the photo is saved (as a jpeg) on your computer, go to your blog. For Blogger you go to: Layout, Header, Image from your computer, upload banner, then click "instead of title and description" under placement. You can use the resize tab in FotoFlexer to change the size to make it fit your blog. Make sure to have the "Keep Proportions" box checked.
If you have any questions, feel free to leave a comment and I'll try to help!