Tools
Each tool means:
REMOVE BG: to remove background color of the image. It could be useful if downloaded png files contain white background.
RESIZE: to resize the image.
TRANSPARENCY: to set the transparency of the whole image.

Remove BG
Seek Bar: Set the tolerance value. Bigger value means remove more similar colors.
Remove Button: Click it to preview the result of background removing.

Resize
New Size: Click on 'Width' or 'Height' to set the new image size.
Resize Button: Click it to preview the result of resizing.

Tools
Seek Bar: Drag it to set the transparency value. Bigger value means more transparent.
Preview Button: Click it to preview the transparency effect.