For products, galleries, and news mentions, don't resize your images. The IndieMade system will automatically create appropriately sized copies of your images for you, so they look great in whatever theme you pick. Switch themes, and your images will automatically get resized to match. This way your site loads quickly and looks great.
If you're adding pictures to a blog post or a page, though, you have more flexibility. Scale them down to whatever size fits your design using your image editor of choice -- this is better than using the rich text editor's scaling functionality, which can result in pixelated images. Further, large images take longer for your audience to download, so smaller is better.
See our photo sizing handbook page for more details.