You'd be surprised at how few people get this. I've seen big blogs who still post individual pictures, then place links beneath them. This might be preference or it might be requested by the people who own the picture, but I have a habit of clicking on pictures I like, if it's a product. This might be because I spend a lot of time on Etsy, or because I make sure my images are links when necessary, but I often find myself clicking pictures on other blogs, expecting to be taken to Etsy listings, but I just end up being taken to a larger version of the image.
This is how you do it, and it's so simple that it's unreal - but, as always, this is what works on blogger.
Upload the image you wish to use in your post as you usually would. Resize and position it as you desire. Once you've done that, you want it to link. I find blogger to be tempramental from time to time, so you might want to continue on and write a few words beneath the image if you intend to continue the post beneath, or just go ahead and finish the post first. Either way, you don't need to, but I do it because once or twice in the past, I've ended up with all the writing afterwards being linked too.
Click the image in your "compose" window of your post which will highlight it and then click "link" at the top, as you would if you were linking text.
Chances are, the picture will have moved a smidge and deselect itself, which is what I think turns people away from trying again. Just click it and then click "link" again. Hey presto, the link window's popped up. Just put the link to the desired location in the appropriate box and save it. That really is it.
I know some of these things might seem stupid, pointless, or even obvious, but I'm writing them based on observations I've made from other blogger users - both big and small blogs do this.
If you've taken several images and arranged them in a nice way on MS Paint and uploaded them all as a single image, then this is, obviously not doable, but I may also point out that I'm also shortly posting a quick tip for positioning images side by side in blogger. Again, it's simple, but I still see people arranging pictures side by side in MS Paint or listing images instead. Again, I understand it's preference, but I will also say that when I was trying to figure out how to do this, the only tutorials I could find told you to use a graph or a table and it was both complicating and not what I wanted. So with any luck, I've helped someone with this. Each of these posts relate to problems I've had with blogger and either had to find help in forums through lack of tutorials elsewhere, or I had to just figure it out myself.