Dealing with multiple images side-by-side is a long-standing and hopefully-someday-will-be-resolved feature request for Blogger, but you can do it pretty easily right now with Blogger in Draft and a tiny bit of playing with Edit HTML.
Upload both photos to Blogger. Leave them centered, but change their size down to small. (You may have a wider post column than mine and be able to leave them at medium.) You’ll want them placed one after the other.
Add a caption to each photo. This is a feature that’s being tested out on Blogger in Draft, so you’ll need to go there to use it. Just click the “Add caption” link in the photo’s tools bubble.
Switch to Edit HTML and find the <table> code. Now things start to get tricky.
Delete the second <table>. You can get rid of the now-empty second table.
Copy the second image’s <td>… …into the first table.