there are various ways to do so, i myself use Paint Shop Pro. For those that do not have this you can also use MS Photo Editor. This is a program that comes standard with either windows or office.

(I am translating the names of functions from Dutch, so the names may be slightly off)

Open the file you want.

Select the menu “Picture” from the toolbar. Open the “resize” function. From here you can resize upwards and downwards. Usually I resize down to 800×600 or 640×480 when posting on this forum, since i have to stay below the 100b.

Now select the “save-as” function. Behind the picture type there is a button with “more”. When you press it you can change the quality factor. The higher the quality the larger the file.