Compressing your images is super important for optimising your websites load time and for SEO purposes.
Step 1 Name your files
Make sure to name your files something that Google will recognise, for example, Simply_Whyte Design_profile_photo.jpg. Do not leave photos as numbers like 456ACG.jpg
Step 2 Check the size of your files
You can do this by changing to list view or right clicking on the image and selecting information. You want to try and get your images under 500KB. An example of a file that is way too large for your website is a file size of 8MB
Step 3 Compress your images
Go to this website https://tinypng.com/ where you can upload images and it will compress them without losing quality. If your images are over 5MB they won't upload. If this happens go to this site https://www.imgonline.com.ua/eng/resize-image.php where you can change the physical size of the image by changing some of the pixels or by decreasing the DPI.
DPI stands for Dots Per Inch. All photos are made up of dots and depending on where you are using your images there are recommended DPI's for every situation. For example, the recommended DPI for web is 72 and for print, it is 300. Print requires more dots in the image to make it more clear whereas on a website you can reduce the amount of dots and the image will be ok to use.
If you have lots of images to do you can open multiple tabs of https://tinypng.com/ and wait for them to compress.
Step 4 Upload your images into Squarespace
Then you can upload your images into your website. If you have a really large gallery with a 100 images try uploading 10-15 images at a time and then once they have uploaded drop some more in. It's easier doing this way because it won't slow down your internet or your websites processing as much.