Drawing multiple Images
Naming

What is the difference between xChunk and chunkX? yChunk and chunkY? Choose names that will help, not hinder you.

Don't skimp on letters. size is better than siz, point is better than kps. Almost anything is better than step2. On the other hand, chunks is better than allChunks, chunk is better than currentChunk.

Class names are usually singular -- see for instance Point, Color, Rectangle. Follow the pattern and rename Chunks to Chunk.

The names Chunk and CeldsChunk don't convey anything to me -- that might be because I have no domain knowledge of what you're doing, or it might be because their names can be improved.

Commented-out code

It's confusing. Is it a failed attempt? Commented out for performance testing? No-one knows. If you might need it later, save it on a new branch. Otherwise, delete it.

Conventions

Follow C# conventions. Install StyleCop and listen to its recommendations.

Performance

Profiling is your best bet. You can try things like moving repeated calculations out of loops, as I suggested in the comment, but it's possible you're just asking too much of the computer. You're drawing up to sizeX * sizeY * ? images, and the solution might be to make do with fewer images.