Use your preferred photo editing tool (GIMP / PhotoShop / PaintShopPro / etc) to adjust the image size (usually using the 'Scale' tool or some variant thereof.
Use 'Save As' to save as a JPEG, choosing a compression ratio to get the best balance between file size and image quality.
Between those two, you should end up with a manageable image size.
There are also utilities such as
FastStone PhotoResizer that will do the same thing without all the image editing utilities