Open your image in GIMP.
Select the area you want to make transparent.Select the appropriate selection tool from the Tool window or the Tools – Selection Tools menu on the Layer window.
I usually use the magic wand/fuzzy select (Select contiguous region) tool or the Select regions by color tool.
Click in the region you want selected. Use shift-click to add more regions/colors.
Tip: It’s easier if you zoom in (View – Zoom menu) to see finer details of exactly what you’re selecting.
In the Layer window (the one showing your image), select Layer – Transparency – Add Alpha Channel. If this is blanked out then it’s already done. This makes sure your image can store transparency data.
Select Edit – Clear. This makes the selection transparent.
Save the file.
Select Edit – Clear. This makes the selection transparent.
Save the file.
Note: If you save it as a PNG file, be sure to select the ‘Save colour values from transparent pixels’ option in the Save as PNG dialog box.