Do you mean using tkinter on your own machine or on the cc workspace?
I don’t know if the workspaces would support tkinter or indeed any GUI even if it was installed (which I don’t think it is, I think they use some fairly stripped back containers hence the fact you cannot import it).
I think this question has been raised before for those looking to use images e.g. for django/flask and the easiest solution wound up being to upload the file elsewhere and make use of the hyperlink (you might find some discussion or preferable alternatives if you search the forums).