Click on the background colour rectangle in the upper half of the Tool Options toolbox window to change the background colour. (In this respect, Adobe Photoshop is no different.) Hence, choose a colour which is compatible with the background on which the resulting picture will eventually be used. Some colour mixing will be inevitable and even desired. Do select BG colour fill and Sample merged and run a few tries while adjusting Threshold for best results. Within the Bucket Fill toolbox options, select Fill transparent areas only when necessary.
Select the bucket fill tool or hit Shift+ B. The next step is optional and consists in filling the area that should become transparent with a plain colour first. This is done by selecting Layer → Tranparency → Add Alpha Channel. If this is the case, add an alpha transparency channel. Some image types lack a transparency channel JPG for example. La trahison des images by fellow countryman René Magritte partly vandalised for the purpose of this tutorial.
