Try the solution proposed by @seankross at seankross/postcards#30 (comment)

Author: lcolladotor

index.Rmd

@@ -10,7 +10,7 @@ output:
   postcards::trestles
 ---
 
-<img src="logos/logo.png" width="500px"/>
+<img src="logos/logo.png" width="100%"/>
 
 **recount3** is an online resource consisting of RNA-seq gene, exon, and exon-exon junction counts as well as coverage bigWig files for 8,679 and 10,088 different studies for human and mouse respectively. It is the third generation of the [ReCount project](http://bowtie-bio.sourceforge.net/recount/) and part of [**recount.bio**](https://recount.bio).