Specifically I want to make a collage of many pictures.
For that purpose the software should be able to handle many large pictures, and align them relative to each other (snap center to center, etc)
I'd like to avoid having to do it in libre office if at all possible.
If you plan on doing it on a regular basis, you might want to try latex. This is going to be hard on the first one or two poster but on the long run it can save a lot of time...
However, citing issues with the open source code being plagiarised by others that had rebranded the software as their own and bundled user content without their permission, the availability of the source code was restricted
In November 2009, the software was made proprietary, restricting the sale or creation of derivative works of the software.
The basic edition is free, and has all the snap to alignments you're looking for. The pro version ($100) lets you save as an SVG, PNG, or PDF, so printing what you make shouldn't be an issue. It's a lot more powerful piece of software than I expected it to be.