Salesforce limits rich text area image uploads to 1 MB to ensure optimal performance and page load times across all users. This limit applies to inline images inserted directly into rich text fields using the Insert Image button.
When you insert an image greater than 1 MB in a Rich Text Area field using the Insert Image button, it will be rejected with the message: "The image exceeded the maximum size of 1 MB."
The maximum size of an image that can be uploaded in a rich text area field is 1 MB. This is a hard-coded limit and cannot be increased. You can upload only .gif, .jpg, and .png file types.
You may also upvote this idea on Salesforce Ideas:
Increase 1 MB limit on Photos inserted into Rich Text Fields
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