thank you for your reply.
i already configured the gridsnap and i can set sprites with staticbody and a collisionpolygon where i want them to be. unfortunately, this didnt solve my problem.
the problem is the following: i convert the scene with the sprites (with their staticbodies and collisonpolygon children) to a tilemap, i make a new scene with a rootnode, set up a tilemap as child and make it get the .tres resource i converted to earlier. so far so good.
when i now want to "draw" with the tileset, i can highlight a square on my canvas but the tile i picked shows up with an offset of 25x25 relative to the mouse. if i click on a square on the grid, the tile gets placed at the offset, instead of where i highlighted. this happens on my pc with win10, steam-version of godot.
i now tried the whole process again on my laptop (ubuntu 16.04) and didnt had any troubles making a tileset and drawing a tilemap with the converted resource. this might be a bug. i'll try re-installing the steam version of godot on my pc and if this doesnt help, i'll try downloading it from the official website and try again.
here is a pic that describes my problem better than i can with my lacking english skills.