Bueno: Remove Image Borders

without a link to your site, from which the css styles would be accessible, direct help is not possible.

general:

in style.css (or any other css files of your theme) look for
img { border: ....; }
which could be in different combinations with other styles.

https://www.w3schools.com/css/css_border.asp

here is the site:

https://www.thewebgents.co.uk/

edit purple.css
(you find this under https://www.thewebgents.co.uk/wp-content/themes/bueno/styles/purple.css if you can’t access it from the dashboard)

and look for:

.entry img { border: 5px solid #d7bced; }

either delete this line, or surround it with comment code, like so:
/* .entry img { border: 5px solid #d7bced; } */

edit:
actually, your theme has a custom.css, in which to enter any css changes that you make:

in your case, enter:
.entry img { border: none; }
to remove the image borders