I personally open the screenshot, than right click it and choose "Open With >> Paint".
I use the "Cut" system in a rectangular mode and select the chat box by drawing it around the chat. Then Copy the new rectangular (make sure you can move it around) and open a new paint page. Paste it there, remove the white borders and you are done.
If that didn't help, watch the following steps in the spoiler:
1) Select your screenshot: (My Documents ---> GTA San Andreas Files ---> SAMP ---> Screens)
2) Right Click ---> Open With ---> Paint
3) Select The Rectangular Cut Mode:
4) Copy the new rectangular (make sure you can move that part) and paste it in a new page