There are a few reasons you might get a mismatch error when uploading images:
1. The file names of your images must match the image file names contained in the products you import.
For example, if you import a spreadsheet that contains the image file named "5678.jpg", the image you import for that product must also be named "5678.jpg". If it's named "6578.jpg" there will be a mismatch and the image will not be imported.
2. You will get a mismatch error if you forgot to indicate which column in your spreadsheet contains your images when you initially imported your products.
3. We are currently investigating a bug that occurs when a user imports an XLS or XLSX file that has multiple image filenames in a single column and separated by a semi-colon. This causes a mismatch error after the files are uploaded.
While we're working to fix this error there is a work-around solution to address this issue:
Convert the XLS or XLSX file to a CSV (In Excel, go to "Save As" and select CSV as the file type) file and re-import your spreadsheet and images.