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If you reduce photos, you lose fine detail, of course.
But: if the photos are not in JPEG format, but in BMP, then just converting the picture files to JPEG or JPG file format, will compress their file-space size or storage size, immensely.
Reducing can be done on any Windows computer, by using the MS Paint or Paint program, which is included in every version of Windows.
First you load the image, then you go to Resize, and where it says 100 and 100, put in 50 and 50, and your picture will lose every other pixel vertically and every other pixel horizontally, and will store in about 1/4 the space perhaps, and certainly contain only 1/4 the pixels, at 1/2 and 1/2 the pixel-width and pixel-height.
Do you need to know how to change the camera settings so that the picture are *taken* at a lower pixel-density, or total pixel number and size?