ガーデニングの本を見てたら、ドクロマークのついたのがいっぱいあって驚きました。

、。

When I was flipping through a gardening book, I was surprised to find lots of things marked with skull symbols.

Literal

Gardening [possessive-の] book [object-を] was-looking-[when-たら], skull-mark [possessive-の] attached [nominalizer-の] [subject-が] lots existed surprised.

~てたら is the casual contraction of ~ていたら — 'when I was (doing X).' ドクロマーク (skull mark/symbol) is a hybrid compound: ドクロ (skull, from 髑髏) + マーク (mark, English). いっぱいあって links the surprise to the quantity. The sentence captures a moment of unexpected discovery.