diff --git a/phpBB/styles/prosilver/theme/content.css b/phpBB/styles/prosilver/theme/content.css
index 0a0e853db2..b99312a5be 100644
--- a/phpBB/styles/prosilver/theme/content.css
+++ b/phpBB/styles/prosilver/theme/content.css
@@ -267,7 +267,7 @@ dd.option {
 }
 
 .postbody img.postimage {
-	max-width: 99%;
+	max-width: 100%;
 }
 
 .search .postbody {
@@ -602,6 +602,7 @@ dl.file dd {
 dl.thumbnail img {
 	padding: 3px;
 	border: 1px solid transparent;
+	box-sizing: border-box;
 }
 
 dl.thumbnail dd {
diff --git a/phpBB/styles/subsilver2/theme/stylesheet.css b/phpBB/styles/subsilver2/theme/stylesheet.css
index 596ebdc29f..3c7ff23db5 100644
--- a/phpBB/styles/subsilver2/theme/stylesheet.css
+++ b/phpBB/styles/subsilver2/theme/stylesheet.css
@@ -632,7 +632,7 @@ input:focus, select:focus, textarea:focus {
 }
 
 .postimage {
-	max-width: 99%;
+	max-width: 100%;
 }
 
 .syntaxbg {