This tip is pretty easy (for blogspot blogs - sorry to other platform users, I'm just not familiar):
Insert a photo into your blog post. You can either upload a photo from your computer or you can upload from a URL. Once the picture is uploaded, click on the photo to highlight it. (You'll get the little white squares in the corner.) Then click on the link button next to the text color button.
When the window opens, paste in the URL (make sure it only has the http:// once, not twice) then click ok.
Ta dah! Your picture is now a link.
Now here's something EVERYBODY should be doing, regardless of blogging skill.
People like clicking on pictures, so make your pictures clickable links as often as possible.
Is this a picture of something you're selling? If you uploaded it from a flickr URL, then it must link back to its flickr page. Make sure it's organized into a set of similar items so people can view your "portfolio," so to speak.
If you uploaded it from your computer or from another URL like Photobucket, then by all means link it to your sales page or shop. Give your readers and viewers every nudge and assistance to make it easy to buy from you.