Make sure that the texture is selected. If you haven't changed the bitmap, select it in the Bitmap Gallery, click the Save button and in the JPEG Export dialog box, select Use Original JPEG option. This dims the other options as the original JPEG is saved out. Otherwise use the Export option on the File menu. In the JPEG tab select a quality setting.
Lower settings give greater compression but lower quality.
You can see how the image looks in the Bitmap Export Preview dialog box. The best setting for the JPEG quality depends on the bitmap. For example the center bitmap at the top of the page has more detail and needed a higher quality setting than the other two. Only you can decide the best compromise between quality and file size. Deselect Progressive JPEG - most browsers don't support progressive rendering of background bitmaps.