add guidance about image sizes

[stefaniebutland]

See Big images issue https://github.com/ropensci/roweb3/issues/6 for suggestion and solution

[steffilazerte]

I can definitely add this to the blog guide.

I plan to add to this section:

• Keep images to < 1 mb (@stefaniebutland @maelle can they be larger? probably not necessary)
• We already suggest using tinyPNG to reduce file sizes

Sound good?

[steffilazerte]

I'll also add a reference to https://bookdown.org/yihui/rmarkdown-cookbook/optipng.html cf (#166)

[maelle]

There's also https://github.com/ardata-fr/minimage by @davidgohel (still experimental, but should be useful later)

I like the ease of use of websites such as tinyPNG but I think using a local tool is probably better?