When attaching an image to a post using the CKEditor you have to click the "Send it to the Server" button. That process sends the image and stores it, by default, in the database (in the qa_blobs table). You can configure Q2A to store the images in a directory of your choice by modifying the qa-config.php file: https://github.com/q2a/question2answer/blob/b0f03e08badc68fa950d615e6450292fa1af6e9b/qa-config-example.php#L85
If you mean where the toolbar images (like bold, underline, etc) are located, you can find them inside qa-plugin/wysiwyg-editor/ckeditor/plugins. There are 2 files: icons hidpi.png and icons.png. There might also be images inside the CKEditor plugin's directories, e.g. qa-plugin/wysiwyg-editor/ckeditor/plugins/smiley/images.
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