Any historian of Cluny is familiar with the Bibliothèque Nationale de Paris, which is home to so many manuscripts from Cluny, and likewise with their online digital library Gallica. An ever increasing number of their manuscripts are now available online, but accessing high quality images can be sometimes frustrating due to the current interface. While the Medieval Manuscripts: France-England 700-1200 initiative shows that the BNF is moving towards an open model of image sharing (using IIIF), the older interface makes copying high resolution images difficult. I was to provide a quick idea of how to work around their interface (and to write up a note so I remember this in the future).
The headers for the Petrus Project blog are taken from a manuscript containing several works by Peter the Venerable (Paris, BNF, ms. latin 17716, which you can consult here). The screen looks something like the following for folio 1r. 
You have the option of downloading the whole manuscript or a selection at a higher resolution. The interface, however, only lets you download a relatively small part of the image. I would suggest instead that you use the sharing function.
This allows you to get a URL for the image to share on a website etc. but you can also use it to download the image at a higher resolution than they want to allow you to. Once you click on the sharing button, you are given the choice to “Copy code for embedding”. Click on “Part of the Image” and then select the portion of the image you want to copy, in the right side of the screen.
I selected a bit of the manuscript, and it gave me the code:
<a href='https://gallica.bnf.fr/ark:/12148/btv1b84274413/f9.image.r=ms' target='_blank'><img src='https://gallica.bnf.fr/iiif/ark:/12148/btv1b84274413/f9/93.21981969935753,141.79358283251105,3434.915483434753,1626.380437232364/264,125/0/native.jpg'/></a>
This is useful for embedding the images in emails, html etc. But I want to just see the image, and a higher resolution. So I delete everything between the first triangle brackets
<a href='https://gall ...nk'><img src='
and the last little bit of the code:
'/></a>
Leaving us with:
https://gallica.bnf.fr/iiif/ark:/12148/btv1b84274413/f9/93.21981969935753,141.79358283251105,3434.915483434753,1626.380437232364/264,125/0/native.jpg
For simplicity I am going to round up the fractional numbers of the last part of the URL.
https://gallica.bnf.fr/iiif/ark:/12148/btv1b84274413/f9/93,141,3434,1626/264,125/0/native.jpg
If we take a look at the address we can break it down into parts:
| Purpose | Part of URL |
|---|---|
| the repository | https://gallica.bnf.fr |
| that the image is stored in a IIIF server | /IIIF |
| the object identifier | /ark:/12148/btv1b84274413 |
| the folio number | /f9 |
| the pixel coordinates of the image selection (making a square from the top left to the bottom right) | /93,141,3434,1626 |
| the number of pixels horizontally and vertically | /264,125 |
| degree of rotation | /0 |
| nature of image | /native.jpg |
If you changed the degree of rotation from 0 to 180, for example, you would turn the image upside-down. If you want to increase the resolution, however, you can increase the numbers of pixels from the current base selection:
to something bigger. If you increase the pixels tenfold:
https://gallica.bnf.fr/iiif/ark:/12148/btv1b84274413/f9/93,141,3434,1626/2640,1250/0/native.jpg
You end up with this, or as much as the WordPress site will show to you:
Hope this can help you!