Update Nov 2018: Google's test is now working properly with Shopify sites. Other sites like Gtmetrix are not working though. 

Important Note: Gtmetrix's Tool does not work properly with Shopify stores. The following guide is unsupported and is a general explanation of what you can try if you absolutely need Gtmetrix's tool to work. 
Use a tool like Pingdom instead for more accurate results. 

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General Information

Although you've used Crush.pics to compress your store images, it's possible that Google's PageSpeed Tool or tools like Gtmetrix are showing you that you need to compress and optimize the images still. 
Although your images have been compressed, the reason these tools see this is that your Shopify theme is not using the correct image resolution. 

For example, your photos might be sized at 300x300 pixels, but your Shopify theme may be pulling that image at 1024x1024 pixels and that would raise a red flag with Google's tool.

The highlighted image is actually 178x145 pixels but Shopify is trying to pull it at 500x406 pixels resulting in a discrepancy and resulting in red flags in Google's tool

"Compressing and resizing … could save…”
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How to solve this

1. Talk to Shopify Support

2. Talk to your theme developer

3. Change your image dimensions and re-upload them > Shopify img_tag Guide

4. Stephen's World has a great guide to solving this problem

5. If you have no time yourself, you can hire the team at speedboostr.com to help you get these all fixed very quickly > https://speedboostr.com/

   1. Joe at SpeedBoostr has also published a post on what you can expect. Check it out!

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Like Joe at SpeedBoostr says in his blog post:

This is a good analysis tool to get ideas for optimizing your site, but it has a few flaws:

1. The scoring system doesn’t correlate to actual speed
2. Sometimes you can make your site faster (or clear warnings) and the score drops (see this case study)
3. If you have 3rd party apps (every Shopify site) you’ll get warnings that can’t or shouldn’t be cleared

I’m not bashing the tool – it’s good to use to get optimization opportunities… just don’t get obsessed with the score.

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You can read more about Shopify image sizes here: https://help.shopify.com/themes/liquid/filters/url-filters#size-parameters

Use Pingdom for best results with Shopify stores