Depending on your photo editing program, size your layout to 600x600 or 500x500 and use the "save for web" feature under File>Save For Web (primarily Photoshop programs, but I believe PSP programs have something similar). You can then slide the size bar along til you get it to just under 150kb. The other thing to make sure of is that you have flattened your layout first. If you're using a layered layout, they are huge.
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