What is procedure to attach image to a post? proper image file format?
Like others have said, I never paste pictures as file attachments using 'Attach files' button, but rather paste them directly into text entry box.
Pictures:
I just select or open any picture file, then hit CTRL-C (COMMAND-C on Mac) to copy it into memory buffer, then click inside the posting window on the forum where I'm typing the post in, and click CTRL-V (COMMAND-V on Mac) to paste (insert) image in, then click on the inserted picture once, and can move mouse pointer to the corner of the image to resize it appropriately.
Screen captures (Mac):
Go to any window or thing on your screen you want to have a screen capture of, on the Mac I hit SHIFT-COMMAND-4, then mouse turns into a target, draw a box around the area you want to capture, right when you let go of the pointer it makes a camera clicking sound-effect and saves the area in the box as picture file to the desktop, then I hit COMMAND-H to hide browser window out of the way, click once on that pic on the desktop (don't even have to open it, just select the icon so it's lit up) and hit COMMAND-C to copy it into the buffer. Then paste into the posting window of forum with COMMAND-V and resize appropriately.
Screen captures (Windows):
For Windows, use the built-in 'Snipping Tool' to make screen caps. Put the window up front you would like to capture the area of. Hit the Windows keyboard key, start typing Snipping Tool and start menu finds it by the time you type sn.., hit Enter to open Snipping Tool. Then click on the 'New' button, and draw a box around the thing you want to capture, release mouse button. Then hit CTRL-C (or go to Edit > Copy), and then it's in the buffer, you can go paste that directly into the posting window in the forum using CTRL-V, and resize appropriately.