Here is a quote from Ashley regarding image sizes:
[quote:1mjjt7dk]Some people struggle with memory usage on mobile devices with Construct 2, but usually from lack of awareness to how images are stored in memory. Your game might be a 5mb download since all the images are compressed as PNG or JPEG, but they are extracted to uncompressed (basically BMP) in memory for rendering, which can easily make it 5-10x bigger in memory than the download size. Because lots of people ran in to this we recently added a memory usage tracker in the Construct 2 status bar. It's just an estimate, but will give you a useful approximate figure (e.g. 5mb download size, 40mb memory use).
Source is Here.
In other words, compressing them is pointless, as they are uncompressed in memory.
More useful info can be found in Ashley's latest blog:
Understanding CPU and memory measurements.
What are your image sizes (largest)?
Thanks a lot zenox98, all my questions have been answered. I learned many things also regarding memory use.
And for your question, the largest image in my game is 400x320 pixels (8kb).
There are 10 objects with 4 animations each. Each animation has 7 frames with that size of image. I put them all in a single layout.