README.md: same size, uploading instructions

README.md

@@ -79,7 +79,7 @@ Themes can be previewed using `Plots.showtheme(thm::Symbol)`:
 ![theme_wong2](https://user-images.githubusercontent.com/16589944/70847880-eaecac80-1e69-11ea-9b59-cdf937427121.png)
 
 ### `:boxed`
-![theme_boxed](https://github.com/user-attachments/assets/40454139-e34d-4c5c-8db5-31a4816bf5f5)
+![theme_boxed](https://github.com/user-attachments/assets/ed1baaf1-4679-4d9a-9667-3118dc42200e)
 
 ### `:gruvbox_dark`
 ![theme_gruvbox_dark](https://user-images.githubusercontent.com/16589944/70847882-f049f700-1e69-11ea-94f8-255f2cd17288.png)
@@ -116,3 +116,10 @@ PRs for new themes very welcome! Adding a new theme (e.g. `mytheme`) is as easy
 _themes[:mytheme] = PlotTheme(; kwargs...)
 ```
 The keyword arguments can be any collection of Plots attributes plus a colorgradient keyword argument.
+For adding the `showtheme` figure to `README.md`, use 
+```julia
+Plots.showtheme(:your_theme)
+plot!(dpi=400)
+savefig("your_theme.png")
+```
+The image can be uploaded to the `README.md` file by pulling it in with the file-editing web interface.