reprex is only setting by default a knitr option called upload.fun to the function in knitr knitr::imgur_upload()
See about upload.fun in https://yihui.org/knitr/options/#package-options
You can provide your own function if you want. knitr::imgur_upload is an example.
You can find another in example for flicker in this old script
You can create a function based on this example: it must take a file in input and return a url in output I guess.
setting this options in a reprex will apply to the reprex code. for example, as shown in ?reprex, you can add
#+ setup, include = FALSE
knitr::opts_knit$set(upload.fun = my_upload_fun)
in your reprex code to change the option. if my_upload_fun = identity for example, no file will be publish to imgur - can be useful for some reprex.
Hope it helps.