Most computers have some kind of photo editing software on them. Most photo editing programs will do both of the following. If you don't have any editing programs on your computer, I recommend IrfanView, which is free, light, and easy to use.
First, reduce the physical size of your photo as measured in pixels. (As it so happens, the moderators are currently discussing adopting a maximum width, so that we don't get the problem of very wide pictures making the text of comments unreadable. We are considering a max width of either 1024 or 800 pixels.) In IrfanView, you would go to Image and then Resize/Resample, then set the Width to 800 pixels. There should be a similar option in most editing software.
If your picture is taller than it is wide, you might want to reduce the height so that it's more likely to fit on most screens. So maybe reduce the height to around 900 pixels.
Next, reduce the quality of, or compress, the picture file. In IrfanView, you do this when you go to save your edited picture. Choose File / Save As; a window pops up where you name the file and choose where to save it, but a second little window pops up at the same time where you can reduce the quality. The default is 100; pull the slider to the left and reduce it to maybe 95. Then save the file. Right click on the file, choose properties, and see what the new file size is. If you're still above 150, try reducing the quality a little lower. You'll quickly get the hang of it.
This quality or compression adjustment will be different in various programs, and perhaps found in different menu settings; but most have it. Usually, reducing the quality from 100 to 90 is not really noticeable in pictures posted to the board.
I hope that helps.
