In order to control site loading times, disk space used, and as a security measure, WordPress limits the file size of any image uploaded to the Media Library. The default maximum size is 2MB. You can change this limit (in php.ini, .htaccess, or wp-config.php) if you wish, but that’s a lesson for another day.
Here are three free online tools for optimizing images that are too big to upload.
1) TinyPNG is my go-to compression tool, and I use it three different ways. First, their online tool offers drag-and-drop compression of both .png and .jpg files up to 5MB in size. You will see this free tool at the very top of their front page.
I also use TinyPNG’s free WordPress plugin (which doesn’t help if your image is too big to upload to your site) but does help optimize of all the various images that WordPress automatically creates from your original (for example, thumbnail, medium, and large image sizes). Even if you optimize your image before uploading it to your Media Library, the images that WordPress creates are not optimized at all; actually, they are quite bloated. But the TinyPNG WordPress plugin helps solve this problem, and you can compress approximately 100 images per month for free. If your needs are greater than that, the costs are quite low: just $0.009 per compression.
Lastly, I also use the $50 TinyPNG Photoshop plugin just for the ease of doing image optimization in Photoshop, which were I do most of my image editing.
2) Compressor.io is another drag-and-drop tool. It offers both lossy and lossless compression, supports four file formats (.jpg, .png, .gif, and .svg), and has a 10MB file limit! Lossy means some degradation in image quality. Lossless means no quality degradation, but less compression. In most cases, you can get away with lossy. If you don’t like the results, you can always go with lossless.
3)Optimizilla optimizes .jpg and .png images, and offers a visual side-by-side comparison of your original and compressed image. With this preview tool, you can modify the quality (and size) of your image with a slider. The results are not instantaneous (it does take a moment for your image to resize) but this is a useful way to decide how much compromise is acceptable on the image quality vs. file size gradient.