If your image is in a picture file format such as jpg, gif, png, tif, then you can open it with Paint, which is a free Windows accessory app.
You can then save it as a .bmp file. In the current version of Paint, when you choose the file menu and click Save As, you may find that the BitMap (BMP) option is not in the drop-down list. So click the 'Other formats' option and at the bottom of the screen, click on the 'Save as type' drop down and select 24-bit bitmap (bmp) from the list of options.
If the picture is part of a pdf, then you may be able to select the image, copy it to the clipboard (Ctrl+C) and then paste it into Paint. Then you can save it as a bitmap. If the definition (pixels per inch) is too low, Paint can resize it. Click on the Resize option in the Home tab of Paint and change the percentage. eg 200 will double the number of pixels in horizontal and vertical directions. Then 'Save as' a bmp.
Alternatively, you can open pdfs in Adobe Photoshop Elements (inexpensive to buy but not free!) then resize and save from Photoshop Elements as a bmp.
Photoshop Elements can also handle other types of picture file and convert them to bmp.
