Note: This answer is based on the fact you don't have any user feedback at this time.
Go with the drop down filter, not the tabs.
As you have rightly said, the benefit of the tabs approach is easier for the user in that it turns a 2-click process, into a 1-click process.
However, this is only really going to be beneficial if the user is expected to change the filter often. I would anticipate that given the purpose of the filter, and the nature of the data (photos), this isn't going to be something the user does often. So saving a click isn't going to matter.
Therefore, we should look at the disadvantages of tabs. For example:
- They can take up a big chunk of screen space, and with an application that is focused on showing lots of images, screen space is in high demand
- They can imply multi-select (as you suggested). Although this can easily be nullified with a good design
I can't think of anymore right now, but the screen space is an important one in this instance. Especially when you consider mobile devices.
So when we compare the frequency of the user saving 1-click, with the extra screen space required by tabs, then the drop down easily wins here (unless usage data later proves otherwise).